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The role of fat in energy balance: de novo lipogenesis THE ROLE OF FAT IN ENERGY BALANCE: DE NOVO LIPOGENESIS
The lipogenic enzymes needed to make new fat are present in humans but in far smaller concentrations than in other animals. The rate of de novo lipogenesis in the liver can be estimated in normal humans and has been uniformly found in the order of only a few grams per day. In other words, even on a high-carbohydrate, low-fat diet, the body would not be expected to turn more than a few grams of the excess carbohydrate into fat.
While the liver may synthesise a small amount of fat from non-fat precursors, other tissues, such as skeletal muscle, may simultaneously be burning fat for fuel. Therefore, net whole body fat synthesis will only be present if the former is greater than the latter, which is rarely the case. Examples of conditions which result in net lipogenesis are forced overfeeding of carbohydrate (i.e. up to 5000kcal of carbohydrate per day in the resting state), some disease states such as cirrhosis or HTV infection, states of malnutrition, direct infusions of simple sugars intravenously (such as during intravenous nutrition) or into the abdominal cavity (such as during peritoneal dialysis). These can swamp the body's ability to use glucose as an energy fuel or store it as glycogen, and under these conditions, some of the glucose is forced to convert into fat. However, this state is very difficult to achieve by eating or drinking the carbohydrate, even in the form of simple sugars.
In 1988, Dr Kevin Acheson and his colleagues from the University of Lausanne in Switzerland showed that drinking 2000kcal of simple sugar solution (500g of dextrin maltose) resulted in only a few grams of fat production, and even massive carbohydrate overfeeding (about 5000kcal/day, 85 per cent carbohydrate) for several days after saturation of glycogen stores resulted in about 150g/day of new fat production.3 Since that time, these results have been replicated through a number of different studies.
Clearly, net de novo lipogenesis requires forced overfeeding of carbohydrate and does not occur under the conditions of ad libitum (at liberty or free) eating in normal individuals. Excessive consumption of carbohydrate (to the level of 50 per cent more than normal), has been shown to lead to increased fat deposition if maintained over the long term (14 days or more), but the indications are that this is because of fat sparing through increased carbohydrate use as an energy substrate, rather than conversion of excess carbohydrate to fat.
Myth-information. Contrary to popular opinion, carbohydrate-rich foods are not as fattening as those high in fat. This is not only because carbohydrates (sugars) contain less energy, and require more energy for storage, but because carbohydrate is not converted to fat in humans under normal physiological conditions.
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